1. Field of Industrial Application
The present invention relates to a numerical control device for a grinding machine to control, for example, the manufacturing of articles having a crowning shape.
2. Prior Art
A conventional grinding machine for grinding a workpiece W having a crowning shape S, is illustrated in FIG. 10. The grindstone table is moved, as shown by the dotted line in FIG. 11, in accordance with predetermined instructions in order to carry out a correction process, of its grindstone 29 by a dresser 32.
The grindstone 29 is fitted so as to be able to rotate on the end of a spindle 30, which is fixed on the grindstone table. The grindstone table is caused to move in the X and Z directions. The circular arc alterations L1 and L3 and the straight line alterations L2 are carried out through a dresser 32 and by means of this the grindstone correction process is carried out.
Furthermore, the movement of the grindstone table is carried out by a conventional feedback control. In accordance with this conventional feedback control, the faster the grindstone table is moved the more lag occurs in the servo system and the adjustment shape therefore becomes disordered, as shown in FIG. 12. This results in problems such as inaccuracies in the grindstone correction process etc.
In other words, a so-called mis-instruction occurs which, as shown in FIG. 12, with regard to the actual movement of the grindstone table (shown by solid lines), is contrary to the correct position command for the servo-motor (shown by the dotted lines). The circular arc alteration L1 of the grindstone table is out of sync with the position command and, although it follows for the straight line alteration L2, it is again out of sync with the position command for the next circular arc alteration L3.
A feed forward control device has been proposed which decreases the lag in the servo system to decrease the disorder in alteration shape.
FIG. 13 is a block diagram of the feed forward control device which is configured as described below.
The feed forward control device is made up of a pulse generator 5 linked to a servo-motor 4. A position converter 6 converts the output from the pulse generator 5 to a present position. A comparator 7 compares the present position output from the position converter 6 and a commanded target position output. A proportional calculator 8 determines from the output of the comparator 7 the amount of servo lag as a proportion of the command speed of the servo-motor 4. A feed forward calculator 9 adjusts the amount of servo-lag on the basis of the commanded target position output. An adding means 10 adds the output of the proportional calculator 8 and the output of the feed forward calculator 9. An analog voltage converter 11 converts the output of the adding means 10 into the servo-motor 4 command speed voltage. A servo driver 12 drives the servo-motor 4 on the basis of the output of the analog voltage converter 11.
Thus, when the target position is determined through a configuration like that given above, the present position of the servo-motor 4 is output to the comparator 7 through the pulse generator 5 and the position converter 6 and the target position and present position are compared in the comparator 7. The results of this comparison are output as an amount of servo lag to the proportional calculator 8 which determines a proportion of the servo-motor 4 command speed which is output to the adding means 10. Further, the target position command is output to the adding means 10 through the feed forward calculator 9 and both of the outputs are added. This addition of the output of feed forward calculator 9 is used to adjust the amount of servo lag. In this adjustment, the amount of servo lag is adjusted through a tuning operation performed by a human operator in order to make the actual grindstone table movement coincide with the position command given to the servo motor.
However, in devices which have such a conventional feed forward control, if the correction process is carried out for grindstones which have a very small amount of change in the X direction as compared with the Z direction, as shown in FIG. 11, even a very small amount of servo lag will have an effect on the grindstone correction shape. In this case, adjustment of the amount of servo lag is carried out by adjusting the feed forward gain. This adjustment of the amount of servo lag is carried out by a difficult tuning operation performed by the human operator, resulting in the expense of much time and effort and inconsistencies due to human error. Moreover, if correction of the servo system takes into consideration such factors as changes in the amount of friction with the passage of time etc., more problems and more work is involved in readjustment of the feed forward gain by the tuning operation.